In a magnetic resonance device (e.g., a magnetic resonance computed tomography system), the body of a person to be examined (e.g., a patient) may be subjected to a relatively high primary magnetic field (e.g., 1.5 or 3 or 7 Tesla) with the aid of a primary magnetic. In addition, gradient pulses are delivered with the aid of a gradient coil unit. High-frequency radio-frequency pulses are then emitted by suitable antenna arrangements via a radio-frequency antenna unit (e.g., excitation pulses), which provides that the nuclear spins of particular atoms resonantly excited by the radio-frequency pulses are tilted by a defined flip angle with respect to the magnetic field lines of the primary magnetic field. When the nuclear spins are relaxed, radio-frequency signals (e.g., magnetic resonance signals) are emitted, received by suitable radio-frequency antennas, and processed further. Finally, the desired image data can be reconstructed from the raw data thus acquired.
For a particular measurement, a particular magnetic resonance sequence (e.g., a pulse sequence) that consists of a series of radio-frequency pulses (e.g., excitation pulses and refocussing pulses as well) as gradient pulses appropriately coordinated therewith to be transmitted in different gradient axes along different spatial directions is therefore to be transmitted. Readout windows chronologically appropriate for this purpose that predetermine the periods of time in which the induced magnetic resonance signals are acquired are set.
With regard to magnetic resonance imaging using a magnetic resonance device, the homogeneity of a primary magnetic field in an examination volume is of importance. Even in the case of small variations in the homogeneity, large variations may occur in a frequency distribution of the nuclear spins, which provides that qualitatively substandard magnetic resonance image data is acquired.
In order to improve the homogeneity in the examination volume, shim arrangements are known. When a magnetic resonance device is installed at a desired location, then fields present in the surrounding area may restrict the default homogeneity of the primary magnetic field (e.g., around an isocenter of the magnetic resonance device). Therefore, during installation and commissioning of a magnetic resonance device, frequently in conjunction with measurements, the shim arrangement is set up such that as optimal a homogeneity as possible is established. Basic shim settings are thereby computed during the installation and commissioning of the magnetic resonance device.
The subject to be examined does, however, itself constitute a further source of inhomogeneity. When, for example, a person to be examined is introduced into the magnetic resonance device, then the matter of the body again disrupts the homogeneity. In order to counter this problem, the use of an adjustable shim unit is known. For example, electrical shim coils are known for this purpose. Electrical shim coils, when driven with different shim currents, generate different compensation magnetic fields in order to improve the homogeneity.
In order to shim the disruptions of the subject to be examined, it is common practice, when driving the shim unit by the basic shim settings obtained during the installation and commissioning of the magnetic resonance facility, to use the magnetic resonance device itself to undertake a measurement of the field distribution when the person to be examined has been introduced into a patient receiving area of the magnetic resonance device. Thereafter, based on the basic shim settings, optimized shim settings are ascertained by a control unit while taking into consideration the measured field distribution. Using the optimized shim settings, the shim unit is then driven in order to achieve as optimal a homogeneity as possible.